Manufacture of electron discharge devices having cathodes



C- H. MELTZER Nov. 2, 1965 MANUFACTURE OF ELECTRON DISCHARGE DEVICES HAVING GATHODES Filed June 27, 1963 INVENTOR C/ML h. MELTZEK United States Patent 3,214,821 MANUFACTURE OF ELECTRON DISCHARGE DEVICES HAVING CATHODES Carl H. Meltzer, Morristown, NJZ, assignor to Radio Corporation of America, a corporation of Delaware Filed June 27, 1963, Ser. No. 291,161 9 Claims. (Cl. 29-2513) This invention relates to electron discharge tubes and particularly to the cathodes of such tubes.

Certain types of electron discharge tubes utilize a cathode comprising a metal sleeve coated with an electron emissive material. A problem long associated with such cathodes is that, for reasons not fully understood, the emissive coating occasionally flakes or peels off the base sleeve during operation of the electron tubes.

It is known that the adherence between an emissive coating and a base sleeve may be improved by providing a sintered metal powder matrix between the sleeve surface and the emissive coating. A sintered matrix is believed to provide a large number of small cracks or openings in which the emissive coating is enmeshed, thereby providing firm attachment of the emissive coating with the cathode sleeve.

The usual cathode sleeve is made of nickel, and nickel is used as the intermediate sintered layer for binding the emissive coating to the nickel sleeve. In the past, a nickel matrix has been provided by applying nickel powder by means of spraying, dusting, or the like, onto the nickel sleeve and sintering the nickel powder onto the nickel sleeve by heating the nickel coated sleeve at an elevated temperature of the order of 1000 C. for a period of to 10 minutes. Thereafter, the emissive coating is applied to the sintered nickel matrix by spraying, dipping, or other suitable means. In some instances, the emissive material is firmly pressed into the nickel matrix. Thereafter, the cathode is assembled into an electron tube and the tube sealed and exhausted.

Although electron tubes known as receiving-type electron tubes have been troubled with the cathode peeling problem for many years, intermediate sintered metal layers between the cathode sleeve and the emissive coating have not been used to solve this problem.

Receiving-type electron tubes generally employ cathode sleeves formed from nickel strip in the order of 2 mils in thickness. Such thin walled cathode sleeves are used to obtain rapid heating and operation of the receiving-type electron tubes after power is applied to the tubes. During assembly of such receiving-type tubes, cathode sleeves having an emissive coating thereon are mounted between a pair of oppositely disposed spacer plates. Holes are provided through the spacer plates for receiving the ends of the cathode sleeves, the fit between the cathode sleeve ends and the holes being relatively snug to provide rigid support for the cathodes.

Sintered intermediate layers have not in the past been used for such receiving-type tubes because the heating of the nickel powder to sinter it, as practiced in the prior art, requires heating the cathode sleeve to around 1000 C., a temperature approaching the melting point of nickel. Such heating of the sleeves tends to anneal and soften the thin walled nickel sleeves thus making mounting of the sleeves between the spacer plates very difficult without warping or bowing the cathode sleeves. Non-receiving type electron tubes which utilize such sintered intermediate layers use thick walled cathode supports and mounting techniques which are not adversely affected by the sintering temperatures.

A further reason for the failure to utilize a sintered intermediate layer in receiving-type electron tubes to improve the adherence of the emissive coating to the cath- 3,214,821 Patented Nov. 2, 1965 ode sleeve is that the provision of such sintered layers, according to prior art methods, is relatively costly. The application of the nickel powder onto the sleeves and the sintering of the powder, usually performed in an oven for a period between 5 to 10 minutes, are added processing steps which add appreciable and undesirable expense to the manufacture of mass produced and generally inexpensive receiving-type electron tubes.

An object of this invention, therefore, is to provide an improved method for providing cathodes having firmly adhering emissive coatings.

Another object of this invention is to provide an improved and inexpensive method for providing cathodes having an intermediate sintered metal layer between the cathode base material and the emissive coating for firmly securing the emissive coating to the base material.

A further object of this invention is to provide a method of fabricating electron tubes wherein the rigidity of thin walled cathode sleeves having an intermediate layer of sintered metal between the cathode base material and the emissive coating is preserved for facilitating assembly of the cathodes into electron tubes.

For achieving these objects, a method is disclosed herein wherein a suitable metal layer, preferably in powder form, is first applied to a cathode sleeve by spraying, dusting, or other suitable means. If the cathode base material is nickel, nickel powder is preferably used. Thereafter, and without prior sintering of the powder onto the base material, a suitable emissive material is applied over the metal powder, by spraying, dipping, or other suitable means.

Part of the emissive material coating becomes enmeshed with the porous powder layer. The metal powder and emissive material coated cathode is then assembled into an electron tube and the electron tube then processed. Such processing normally comprises evacuating the electron tube, heating the tube metal parts for driving gases therefrom, and heating the cathode at elevated temperatures for activating the emissive coating. During the high temperature heating of the cathode, the metal powder becomes sintered to the cathode base material and a firm bond is provided between the emissive material and the sintered intermediate layer.

Advantages of this method are that since the metal intermediate layer is not sintered prior to assembly of the cathode into an electron tube, the cathode base material is not softened prior to its assembly into an electron tube. Further, no separate sintering operation is performed, hence, the processing time required in the manufacture of the electron tubes is not increased. Further advantages will appear hereinafter.

In the drawing:

FIG. 1 is a view in perspective of an indirectly heated cathode mounted between a pair of spacer plates with a part of the cathode broken away; and,

FIG. 2 is an enlarged cross section of a portion of the cathode shown in FIG. 1.

FIG. 1 shows an example of an indirectly heated cathode 10 comprising a nickel sleeve 12 on which is coated a material 14 convertible upon suitable processing into an electron emissive material. Material 14 may comprise a mixture of alkaline earth carbonates. The cathode 10, as shown, is mounted between a pair of spacer plates 16 and 18, usually of mica, which also support other electrodes (not shown) to form an electrode cage which is sealed within a suitable glass envelope. In order to provide firm support for the cathode 10, the ends of the cathode are received in snug fit through holes through the micas. The Wall of the cathode is thin, that is, about 3. or 4 mils and less, and it is thus desirable that the cathode material be rigid and stiit to permit insertion of the ends of the cathode through the mica holes without distorting the thin walled cathode. A heater 20 is contained within the cathode 10 for heating the cathode. Means, not shown, are provided for making electrical connections with the legs 22 of the heater 20. For providing a firm bond between the carbonate coating 14 and the sleeve 12, a bottom layer 26 (FIG. 2) of nickel powder is provided which is sintered to the nickel sleeve 12 and which, with its rough porous surface, provides a bonding layer in which a portion of the carbonate coating 14 is enmeshed.

In the preparation of cathodes 10 a nickel strip may be formed by suitable and known means into the tubular shape shown in FIG. 1. Thereafter a thin layer of nickel powder is applied to the surface of the nickel sleeve. The layer of nickel powder is porous in order that the subsequent coating of alkaline earth carbonates may become enmeshed therein. A preferred and inexpensive way of applying the nickel powder is by conventional spray techniques, although other means such as dusting, cataphoretic coating, and the like, may be used.

An example of a spray suspension which may be sprayed onto the sleeve comprises a suspension of carbonyl nickel powder in a binder consisting of a 2% dispersion of nitrocellulose or methyl methacrylate in a solvent or mixture of solvents consisting mainly of butyl acetate and ethyl alcohol. The binder, in one series of tests, was made to a density of 0.876 to 0.883, with a viscosity of 21.5 to 23.5 centipoises at 23 C. The nickel powder had a bulk or packing density of between 0.8 and 1.1, and had a particle size between 2 and microns. The suspension of nickel powder had a density of 1.0 at 23 C. The powdered nickel layer sprayed onto the sleeve may be between 0.00025 and 0.0025 inch, and preferably between 0.00050 and 0.001 inch. The thin nickel powder layer is porous, and for a thickness of 0.0010 inch, may have a weight of around 0.5 milligram per square centimeter of coated cathode surface.

After this first powdered nickel layer has been applied, the nickel-powder coated cathode may be air dried in an oven for one minute at 150 C. in order to evaporate the solvents and leave the carbonyl nickel powder adhering to the cathode sleeve surface with the nitrocellulose, methyl methacrylate, or other conventional organic binders holding the powder in place. Since conventional sprayed cathodes for receiving-type electron tubes are similarly air dried in an oven, this step does not cause an increase in the cost of cathodes.

Thereafter, the nickel-powder coated sleeve 12 is coated with the alkaline earth carbonate coating. The alkaline earth coating may be applied, for example, by spraying or dipping.

The hypothetical appearance of the two coatings on the cathode sleeve 12, prior to the processing of the cathode 10, is shown schematically in FIG. 2. Due to the porosity of the nickel undercoating 26, shown as the dark particles 26, portions of the overcoating 14 of carbonates, shown as the light particles 14, penetrate into the nickel undercoating 26 and become enmeshed therewith. Because of the penetration of the carbonate coating into the nickel powder coating and the thinness of the nickel powder coating, generally about 1 mil in thickness, substantially the same amount of carbonates may be applied to nickel powder coated cathode sleeves as would normally b applied to these cathode sleeves when non-nickel coated.

As shown in FIG. 2, the nickel powder layer 26 is completely covered by the carbonate layer 14. If the nickel powder coating were not completely covered by the carbonate coating and extended to the surface of the coated cathode, the thermal color or thermal emissivity of the nickel-powder coated cathode would be greater than conventional or non-nickel-powder coated cathodes due to the darkening of the cathode surface by the nickel powder. Such increase in thermal emissivity is undesirable because the heat loss from a cathode is a function of its thermal emissivity, and the greater the heat loss, the less 4- eflicient is the electron tube using the cathode. For a cathode having an increased thermal emissivity, the heater wattage input to the cathode would have to be increased in order to compensate for the radiation losses.

After the cathode sleeve 12 has been coated with the nickel powder and overcoated with alkaline earth carbonates, the cathode 10 may be assembled into an electron tube assembly by being mounted along with other electrodes (not shown) between the spacer plates 16 and 18 (FIG. 1). A tight fit between the ends of the cathode sleeve 12 and the spacer plates is provided in order to provide rigid support of the cathode 10 to prevent rattle thereof. Heater 20 is inserted into the cathode 10, and the legs 22 welded to suitable connection pins. Thereafter, the cathode 10 and associated electrodes and heater 20 are assembled into a glass envelope and processed into a finished electron tube. The processing of tubes having nickel-powder coated cathodes may be performed in the same manner as the processing of receiving-type electron tubes having conventional non-nickel-powder coated cathodes.

Such processing is usually performed on what is known as a sealex machine, which includes means for sealing the glass envelope around the electrode assembly, heating the tube metal parts to driv gasses therefrom, energizing the heater of the indirectly heated cathode for heating the cathode, and simultaneously exhausting the electron tube through an exhaust tubulation. During the heating of the cathode, the alkaline earth carbonates are first broken down to alkaline oxides which are then further reduced to alkaline earth metals, as known. Also, during the heating of the cathode, the undercoating 26 of nlckel powder is sintered onto the surface of the cathode sleeve and becomes firmly bound therewith. Since the emissive coating 14 is enmeshed with the nickel powder, the emissive coating also becomes firmly bound to the cathode sleeve. During processing of receiving-type electron tu-bes, it is usual to heat the cathodes to a temperature of about 1000 C. This is the temperature which is used in the pr or art for sinterin-g nickel powder to nickel sleeves prior to the assembly of the cathode sleeves into the electron tube assemblies. The sintering step of the prior art, however, usually required a period from 5 to 10 minutes. I have discovered, however, that sintering of the nickel powder to the cathode sleeve may be accomplished during the normal high temperature heating of the cathode during processing of the electron tube on the sealex machine. Such high temperature heating of the cathode usually lasts for as little as between 25 and 60 seconds, depending upon the particular electron tube type, and upon the speed of operation of the sealex machine.

Further, in some instances, it is desirable to process the cathodes of receiving-type electron tubes at a temperature much lower than 1000 C., and in the order of 800 C. Such low temperature processing of the cathode is done to minimize vaporization of the electron emissive material from the cathode. The vaporized emissive material condenses on cooler electrodes and contributes to undesirable electron emission from these electrodes. However, I have discovered that the nickel-powder undercoating may be adequately sintered to the nickel cathode sleeve at temperatures in the order of 800 C. while still providing excellent adherence of the emissive coating to the cathode sleeve.

Although the method of this invention has been described in connection with indirectly heated cathodes made of nickel, the invention may be used in the manufacture of other tyeps of cathodes. Directly heated cathodes, for example, either in ribbon or sleeve form, and made from nickel alloys, such as an nickel, 5% iron, 15% chromium alloy known as Inconel, may be provided with a nickel intermediate sintered layer between the base material and the emissive coating. Other examples will be apparent to those skilled in the art.

What is claimed is: 1. The method of making an electron tube comprismg:

applying a layer of metal powder onto a cathode base member, applying an electron emissive material onto said metal powder layer, assembling said coated base member into an electron tube assembly, and thereafter heating said base member for first sintering said metal powder onto said base member. 2. The method of making an electron tube comprismg:

applying a porous layer of nickel powder onto a nickel base member, applying an electron emissive material onto said nickel powder layer, assembling said coated base member into an electron tube assembly, and thereafter heating said base member for first sintering said nickel powder onto said base member. 3. The method of making an electron tube comprising: applying a porous layer of metal powder onto a thin walled cathode sleeve, applying an electron emissive material onto said metal powder layer, snugly assembling said coated sleeve between a pair of spacer plates, and thereafter heating said base member for first sintering said metal powder onto said sleeve. 4. The method of making an electron tube comprising: applying a porous layer of metal powder onto a cathode base member, causing an electron emissive material to become enmeshed with and to completely cover said metal layer, assembling said coated base member into an electron tube assembly, and thereafter heating said base member for first sintering said metal powder onto said base member. 5. The method of making an electron tube comprismg:

applying a porous layer of metal powder onto a thin walled cathode sleeve, causing an electron emissive material to become enmeshed With and to completely cover said metal powder layer, mounting opposite ends of said sleeve between a pair of spacer plates and in snug fit therewith, and thereafter heating said base member for first sintering said metal powder onto said base member.

6. The method of making an electron tube comprismg:

providing a porous layer of nickel powder on a thin walled nickel sleeve,

applying a coating of alkaline earth carbonates onto and covering said nickel powder layer,

engaging opposite ends of said sleeve with spacer plates during the assembly of an electron tube and prior to sintering of said nickel powder, and

heating said sleeve to a temperature around 800 C.

for a period of not more than seconds for sintering said nickel powder to said sleeve and for process ing said carbonates.

7. The method of making an electron tube comprising:

spraying a suspension containing nickel powder onto a thin walled nickel sleeve for providing a porous layer of nickel powder on said sleeve,

spraying a suspension containing alkaline earth carbonates onto and covering said nickel layer,

snugly engaging opposite ends of said sleeve with spacer plates during the assembly of an electron tube without prior sintering of said nickel powder, and

heating said sleeve for sintering said nickel powder to said sleeve and for processing said carbonates.

8. The method of making an electron tube comprising:

providing a porous layer of nickel powder on a thin walled nickel sleeve,

causing a coating of alkaline earth carbonates to become enmeshed with and to cover said nickel powder layer,

snugly engaging opposite ends of said sleeve with spacer plates during the assembly of an electron tube without prior sintering of said nickel powder, and

heating said sleeve to a temperature of about 800 C.

for a period of not more than 90 seconds for sintering said nickel powder to said sleeve and for processing said carbonates.

9. The method of making an electron tube comprising:

spraying a suspension containing nickel powder onto a thin Walled nickel sleeve for providing a porous layer of nickel powder between 0.00025 and 0.0025 inch thick on said sleeve,

spraying a suspension containing alkaline earth carbonates into and covering over said nickel powder layer,

snugly engaging opposite ends of said sleeve with spacer plates during the assembly of an electron tube, and

heating said sleeve to a temperature in the order of 800 C. for a period of not more than 90 seconds for first sintering said nickel powder to said sleeve and for processing said carbonates.

No references cited.

RICHARD H. EANES, JR., Primary Examiner. 

1. THE METHOD OF MAKING AN ELECTRON TUBE COMPRISING: APPLYING A LAYER OF METAL POWDER ONTO A CATHODE BASE MEMBER, APPLYING AN ELECTRON EMISSIVE MATERIAL ONTO SAID METAL POWDER LAYER, ASSEMBLING SAID COATED BASE MEMBER INTO AN ELECTRON TUBE ASSEMBLY, AND THEREAFTER HEATING SAID BASE MEMBER FOR FIRST SINTERING SAID METAL POWDER ONTO SAID BASE MEMBER. 